1. Technical Field
The invention relates to extruded materials and in particular to extruded elastomeric materials, such as rubber compound for use in the formation of tread strips and side wall strips for use in the manufacture of pneumatic tires. More particularly, the invention relates to a screen assembly for removing foreign particles from rubber compounds being discharged from a usual extruder prior to forming the extruded rubber compound into the desired formation for use in a tire manufacturing operation.
2. Background Information
In the manufacturing of vehicle tires, one of the major ingredients is elastomeric material or "rubber" which is obtained from various sources and compounded with other ingredients. This rubber compound is ultimately placed in usual screw-type extruders, where it is heated and forced through the extruded discharge, afterwhich it is generally extruded into sheets or strips of the rubber compound, or in certain applications, directly into a cylindrical configuration for subsequent use in the tire manufacturing procedure. This rubber compound usually contains a variety of foreign particles and materials depending upon its place of origin which may range from pieces of burlap sack, wood chips, nuts and bolts, hard metal flakes, and the like. These foreign materials must be removed to ensure that the final extruded product is sufficiently free of such materials before being incorporated into the tire manufacturing process.
It is a common practice in various industries, but not in the tire manufacturing industry, to remove such foreign materials, by passing the material through screen assemblies which effectively strain and remove the foreign particles from the material passing therethrough. Many screw extruders designed for thermoplastics are supplied with breaker plate screen supports which are simply flat plates of high strength steel with a plurality of holes drilled through them. Screen packs which are mounted on such breaker plates, are often used in plastic extrusion to create back pressure in the extruder in order to stabilize pumping rates and enhance mixing in the extruder screw. Thus, this creation of back pressure in the plastic extrusion industry is beneficial during the extrusion step and therefore there has been very little use or need for low pressure screening systems.
Perforated dome-shaped screen supports are available for screening rigid PVC material enabling them to be made slightly thinner and to have less pressure drop than flat plates. Also automatic screen changers are popular in certain industries in which one or two breaker plates are mounted in a sliding shuttle, whereas in other installations, a belt of wire fabric is drawn across a stationary support plate to provide the screening element adjacent the extruder discharge.
The synthetic fibers industry has adopted "extended area" filtration systems, wherein filter screens and/or felts can be wrapped around a perforated pipe support, in which the filter medium can be pleated, or a powder can be sintered to form a porous tube. These structures are sometimes combined, but flow is usually inward. Because of the extreme filtration required and nearly Newtonian rheology of nylon or PET polymers, pressure drop through such supports is much less than through the filter medium. Also, screen changing and cleanup is much more complicated than for the plastics industry discussed previously.
Screen basket type assemblies also have been used, mainly in the chemical and food processing industry. Basket strainers comprised of a 20 mesh/inch filtering basket mounted inside a two to four mesh support basket, have been used in experimental demonstrations of low pressure straining of rubber compounds. However, it was found to be impractical to fabricate a sufficiently strong support basket of good quality and proper dimensions. Heavy gauge wire cloth which work-hardens in weaving resists subsequent rolling and welding operations to form the basket and annealing the wire cloth would destroy its strength. Custom adapters had to be machined to fit each support basket to the extrusion head. Also, recovery of used baskets proved to be an even greater problem because rubber could not be removed, nor could the filtering and support baskets be separated. These experimental basket filters were recovered by burning the rubber away from the basket, but this is environmentally unacceptable and costly for production.
Many types of elastomeric material or rubber compounds are extruded from large extruders at rates generally between 5,000 lbs/hr. and 15,000 lbs/hr. without screening. In those applications requiring screening, extremely thick breaker plates are needed to resist the pressure force which is exerted over the large diameters, which increases back pressure. Rubber compound extruder screws usually are designed more for high output than high pressure, so that extra pressure causes intense back mixing. Unless extruder output is drastically reduced typically by 40% to 50%, the back pressure will generate excess heat within the stream of elastomeric material which can scorch the rubber compound.
One common type of screen assembly intended to reduce back pressure in a rubber compound extruder consists of several relatively small thin breaker plates and a large thick support plate. For example, seven 2.5 inch diameter breaker plates are set into the counterbores of holes through a 10 inch diameter support plate, three inches thick. However, the use of such support plates with a plurality of circular holes formed therein, requires an extremely thick screen support plate in order to withstand the high internal shear and bending stresses exerted thereon. This thickness increases considerably the back pressure within the incoming stream of rubber compound, thereby increasing the scorching problem requiring reduced output to avoid such scorching.
Another problem with the screening of elastomeric material is that start-ups with cold stiff rubber, generate extreme high internal pressures, so the screen assembly must be safe from rupture possibly up to 2000 psi.
Therefore the need exists for a screen assembly for screening elastomeric materials which is able to withstand relatively high pressures which are exerted thereon by the moving stream of elastomeric material, at the time of start-up, without creating an excessive back pressure and without reducing the usual output rate of the extruder.